Field of the Invention:
The invention relates to an apparatus for switching a direct current in a high-voltage line with a large number of switching units which are arranged to form a series circuit in the high-voltage line. Each switching unit in this case comprises a switching element as well as a surge arrester arranged in a circuit parallel with the switching element, the threshold voltage of said surge arrester being greater than a rated voltage of the switching element, wherein the sum of the rated voltages of the switching elements corresponds to at least an operating voltage of the high-voltage line.
High voltage refers in this context to voltage potential differences of at least 10 kV, for example with respect to ground potential.
An apparatus of this sort is known from WO 2011/141054 A1. The switching elements according to WO 2011/141054 A1 are semiconductor switches. In order to keep the forward conduction losses that arise in the apparatus during a normal operation as low as possible, a further switching unit which comprises a mechanical switch is arranged in series with the switching units with the semiconductor switches. In normal operation, the current flows through the mechanical switch, whose losses are small in comparison with those of the semiconductor switches. When the known apparatus is switched off, the semiconductor switches are switched off first, after which the mechanical switch can be triggered and the current in the high-voltage line interrupted.
The known apparatus is provided for use in a high-voltage line that can have an operating voltage with a magnitude of up to several hundred kilovolts. For that reason, the known apparatus must comprise a large number of semiconductor switches which first limit the current, so that the mechanical switch can be triggered. A large number of semiconductor switches, however, results in losses in the apparatus that are still high. The switching time of the apparatus also lengthens when the semiconductor switches and the mechanical switch operate in temporal sequence. The use of semiconductor switches, furthermore, leads to high manufacturing costs for the apparatus.